1. Field of the Invention
The present invention relates to a method for processing myoelectric signal data. More particularly, the present invention relates to a method for separating signal data representative of spinal and supraspinal signal components of myoelectric signals.
2. Description of the Prior Art
Electromyographic signals (EMG or myoelectric signals) are used to gauge the contraction of muscles during skeletal movements. Muscle tension has been found to be monotonically related to the amplitude of rectified and integrated EMG signals. Much research has been directed towards refining the EMG signals for use as a source of control information for a prosthetic extremity.
There are traditionally two methods of acquiring EMG signals. With the first method, the EMG signals are typically sampled over long time intervals and are not averaged. The EMG signals acquired through this method are referred to as long latency, unaveraged EMG signals. Their signal character is one of an amplitude modulated, random noise carrier signal and is defined by the equation: EQU e(t)=m(t)*n(t) Eq. 1
where e(t) is the long latency EMG signal as a function of time, m(t) is a modulation function related to muscle contraction and n(t) is a random noise carrier.
Processing of e(t) to provide an estimation of muscle shortening or torque production requires demodulation (i.e., use of a nonlinear element), generally taking the form of rectification and low pass filtering, to produce m(t). The demodulated signal, m(t), can then be averaged over a number of repetitions if an external trigger event is present to increase the signal-to-noise ratio for m(t). If e(t) is not demodulated, and if e(t) is averaged over a sufficient number of repetitions, e(t) approaches a zero level signal. This result is verified by published experimental observation.
With the second method, EMG signals are acquired from a muscle subjected to a rapid stretch or from a muscle subjected to any other means of activating the stretch reflex (e.g., electrical stimulation). The EMG signals acquired through use of the second method are used to assess the function of spinal and supraspinal (cortically-based) reflex. With the second method, the EMG signals are sampled over a short duration, usually less than one second, and are averaged over many repetitions triggered by the onset of a mechanical or electrical trigger. The EMG signals acquired through the second method are referred to as short latency, averaged EMG signals.
Traditionally, the signal processing of short latency, averaged EMG signals has taken the same form as the signal processing for long latency, unaveraged EMG signals (e.g., demodulation by rectification and low pass filtering before averaging). Implicit in using this processing scheme is the acceptance of equation (1) as an accurate mathematical representation of the short latency, averaged EMG signals. In other words, using demodulation before averaging in the signal processing scheme for short latency EMG signals reflects the traditional thinking that failure to demodulate before averaging will result in a zero level signal after averaging a sufficient number of cycles.